Conventionally, in the art described, for example, in the publication of JP-A 10-47469, torque-reduction control is carried out to enhance the shift quality of the automatic transmission mounted on the vehicle and the endurance reliability of a friction material for shifting.
This torque-reduction control is designed such that torque for producing a speed change is increased by a reduced portion of engine torque to allow completion of shifting in a short time. Moreover, a load is decreased by a shortened portion of a shift time to allow enhancement in the endurance reliability.
This is carried out, for example, as shown in a flowchart in FIG. 10 for illustrating torque-reduction control and a time chart of a gear ratio and a torque-reduction amount in FIG. 11. Specifically, after a lapse of a predetermined time T1 from output of a shift command, engine torque is reduced gradually. And when an actual gear ratio becomes smaller than a third set gear ratio G3, engine torque is regained at a stroke to allow shortening of a shift time.
In the above-mentioned prior art, however, torque is sustained during the predetermined time T1, then reduced gradually. Thus, if a torque reduction is started with the actual gear ratio not changed, acceleration is too reduced by a portion of torque reduction, leading to a problem of occurrence of a torque falloff feel. On the other hand, torque is sustained during the predetermined time T1, so that if a start timing of torque reduction is too delayed, progression of the inertia phase is delayed, leading to a problem of elongation of shifting.
Moreover, at completion of torque-reduction control, torque is regained at a stroke when the third set gear ratio G3 is established. However, if a torque reduction is completed before completion of the inertia phase, progression of the inertia phase is delayed, leading to a problem of elongation of shifting (a shift time is elongated) or impossible completion of the inertial phase due to turning-back of shifting.